1. Field of the Invention
The present invention relates to an engine rotation speed control system and more particularly to an engine rotation speed control system for internal combustion engines which will ensure a predetermined speed of engine rotation during idling by supplying sufficient air to the intake manifold through solenoid actuated valve means.
2. Prior Art:
It is well known that during idling of an internal combustion engine, the low rotational speed thereof is subject to wide fluctuation due to the effect of various external conditions.
In the past, an engine rotation speed control system has been proposed in order to stabilize engine rotation speed by supplying the required amount of air to the intake manifold downstream of the throttle valve which is substantially closed during idling. Such an engine rotation speed control system is equipped with a solenoid-actuated valve having a solenoid coil so that the air volume to be supplied to the intake manifold is increased or decreased by increasing or decreasing, respectively, the strength of the electrical current flowing in the solenoid coil. In other words, the air volume is in direct linear proportion to the strength of the electric current. Such an engine rotation speed control system is also equipped with a control unit which determines the optimum strength for the electrical current flowing in the solenoid coil by reflecting various conditions such as the temperature of the engine cooling water or actual engine rotational speed on the assumption that the voltage of the battery which supplies electrical current to the solenoid coil is constant. Thus, the air volume to be supplied to the intake manifold downstream of the throttle valve is determined in accordance with the various conditions as mentioned above under the assumption of constant battery voltage. However, it is well known that the voltage of a battery will drop at cold temperatures. Therefore, the foregoing conventional engine rotation speed control system suffers the drawback that the required volume of air is not supplied to the intake manifold downstream of the throttle valve through the solenoid-actuated valve since the solenoid-actuated valve would not receive sufficient current.